Radio-Frequency Identification (RFID) systems typically include RFID readers, also known as RFID reader/writers or RFID interrogators, and RFID tags. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio-Frequency (RF) wave performs the interrogation. The RF wave is typically electromagnetic, at least in the far field. The RF wave can also be predominantly electric or magnetic in the near field. The RF wave may encode one or more commands that instruct the tags to perform one or more actions.
A tag that senses the interrogating RF wave may respond by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways.
The reflected-back RF wave may encode data stored in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on. Accordingly, when a reader receives tag data it can learn about the item that hosts the tag and/or about the tag itself.
An RFID tag typically includes an antenna system, a radio section, a power-management section, and frequently a logical section, a memory, or both. In earlier RFID tags the power-management section included an energy storage device such as a battery. RFID tags with an energy storage device are known as battery-assisted, semi-active, or active tags. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. Such RFID tags do not include an energy storage device, and are called passive tags. Of course, even passive tags typically include temporary energy- and data/flag-storage elements such as capacitors or inductors.
A goal of many RFID systems is for a reader to inventory all of the tags in a population. Typically, a reader inventories a first tag, instructs the tag not to reply for a period of time (a persistence time), and then proceeds to inventory a second tag. In an ideal world, the reader would continue the process until it has inventoried all of the tags. Unfortunately, if the reader has not finished inventorying all of the tags within the persistence time then the first tag will forget that it has already been inventoried and will allow itself to be inventoried again. Subsequently, the second tag will exhibit the same behavior, followed by later tags. If the tag population is very large then the reader may spend so much time re-inventorying already-inventoried tags that it cannot spend enough time searching for the hard-to-inventory tags. As a result, some of the tags in the population may not be inventoried at all.
One solution is for the tags to have very long persistence times. However, this solution is problematic because if an inventoried tag leaves the field-of-view of a first reader and enters the field-of-view of a second reader before its persistence time has expired, then it will not respond to the second reader until the end of its persistence time, and may actually leave the field-of-view of the second reader before it is inventoried at all. In this two-reader scenario it is therefore desirable for tags to have short persistence times. The conundrum is that in some situations it may be desirable for a tag to have long persistence times, whereas in other situations it may be desirable for tags to have short persistence times. Currently, tags have persistence times that are chosen at the time of manufacture and cannot be changed afterward. These chosen persistence times are at best a compromise and not ideal for many inventory situations.